Drawing Quality steel has, of course, good drawability usually derived from the low carbon content of the steel. In one current practice of producing Drawing Quality steel for galvanized sheet to be used for example, in the automotive industry, the steel upon production is poured into ingots and slabs are formed at an ingot-to-slab efficiency of about 90%. If the steel were continuously cast, rather than poured into ingots, the efficiency could be improved to 95%. In addition, it would result in better drawing performance because often the ingot product suffers breakage in the draw press due to localized stringer-type inclusions which are far less prevalent in a low oxygen content, continuously cast steel product.
It is now known that in order to continuously cast steel, the oxygen content must be sufficiently low, below 125 parts per million, and preferably below 100 parts per million to avert formation of blow holes at and near the surface of the continuously cast slab, which blow holes result in surface defects, such as lines and slivers in the sheet product to be formed. It therefore has been necessary to deoxidize the steel such as through the addition of aluminum and/or silicon such as, for example, disclosed in my U.S. Pat. No. 3,702,243, issued Nov. 7, 1972 and No. 3,793,000, issued February 19, 1974 and both assigned to the assignee of the subject application; and U.S. Pat. No. 3,459,537 to Hornak, issued Aug. 5, 1969. However, it has been found that certain additives of the prior art create a problem in achieving the required grain size of 8 or 9 for Drawing Quality steel to be treated in a continuous galvanizing line. Also the use of silicon as a deoxidizing agent is undesirable for producing the Drawing Quality steel of the present invention since it imparts a hardness to the steel which may adversely effect its drawability. Theoretically, this problem could perhaps be overcome if higher temperature heat zones and slower line speeds were employed in the continuous annealing process. However, modification of existing continuous annealing apparatus to achieve that end would not be feasible, particularly since it would result in decreased production as well as maintenance problems due to the requirement of slower line speeds and increased heat.
The discovery of the present invention is that the above problem may be overcome through the composition of the steel and without requiring modification of existing commercial continuous annealing apparatus. It has been discovered that in order to achieve proper grain size and other physical properties required by Drawing Quality steel, the steel must have exceedingly low carbon and aluminum contents, namely less than 0.02% carbon and less than 0.023% aluminum and preferably less than 0.015% aluminum. Moreover, in instances where the steel is to be used for making galvanized sheet, silicon should not be included in the deoxidizing additives since it would render the steel too hard for Drawing Quality applications.